Criteria for Determining a First-Degree Heart Block
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Article at a Glance
- First-degree heart block is characterized by:
- Regularity: Depends on underlying rhythm
- Rate: Depends on underlying rhythm
- P Wave: Uniform with one for every QRS
- PR Interval: > 0.20 seconds and consistent
- QRS Complex: < 0.12 seconds
Criteria
We must examine the relationship between the P wave and QRS complex. A normal PR interval (PRI) is between 0.12 and 0.20 seconds. A first-degree heart block occurs when the PR interval is greater than 0.20 seconds.
The second largest box on an ECG paper measures 0.20 seconds. Locate a P wave that starts right on that box, and if the QRS occurs after the beginning of the next box, there’s a first-degree heart block.
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The QRS complex occurs after the beginning of the second box, so we know the PRI is greater than 0.20 seconds.
A first-degree heart block is usually benign and asymptomatic. You should still keep an eye on it, as it points to a conduction problem somewhere, potentially around the AV node.
Summary
Physicians must interpret the patient’s ECG grid to diagnose a potential heart block, paying close attention to the relationship between the P wave and QRS complex. The PR interval is greater than 0.20 seconds during a first-degree heart block. A first-degree block is typically benign, but it may signify a conduction problem around the AV node.
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